The report, published Thursday by the Centers for Disease Control and Prevention (CDC), focused on a measure called total fertility rate. That's the number of expected births a group of 1,000 women would have during their lifetimes, according to current age-specific birth rates.

In order to sustain a population, a country needs a total fertility rate of 2,100 births per 1,000 women, the report said.

But in 2017, the US rate was 1,765.5 per 1,000, or 16% below what's required to replace the population over time. (There were some wide swings between states, however: South Dakota had the country's highest total fertility rate at 2,227.5, and the District of Columbia had the country's lowest at 1,421.0)

Experts suspect there are several forces behind the trend

In its report, the CDC did not say why the total fertility rate had declined, but health experts have offered some explanations for the trend.

In an interview with NBC News, Dr. John Rowe, a professor at Columbia University's Mailman School of Public Health, said shifts in women's societal role is one factor.

"In general women are getting married later in life," he told NBC News. "They are leaving the home and launching their families later."

Rowe added that the growth of sex education has resulted in fewer teen pregnancies.

"We've been seeing, year after year, a precipitous drop in the number of births to teenage girls," he told NBC News. "That's good news."

Other experts have cited economic factors, like the 2008 recession and high education costs, as factors behind declining birth rates, as Business Insider reported last year. And in 2018 survey conducted by The New York Times, adults who want kids said they sometimes end up having few, or zero, kids due to the high cost of childcare.

Birth rates have long been below "replacement level," fueling fears of a "demographic time bomb."

Some fear the US could become a "demographic time bomb." Angel Valentin/Getty Images

Since the 1970s, birth rates in the US have been below "replacement level," or the rate at which new births keep the population steady.

This trend has sparked worries that the US may be headed for what's known as a "demographic time bomb," in which an aging population isn't replaced by enough young workers.

"I think the concern is — and there is a concern — is having a fertility rate that doesn't allow us in effect to perpetuate our society," Dr. Georges Benjamin, executive director of the American Public Health Association said in an interview with CNN about the new CDC report. "We may very well over time start seeing this reversed or flattened out, but that remains to be seen."